Re: A lean way for getting the size of the instruction at a given address

Luis Machado via Gdb
2021-04-05T16:40:29Z

On 4/5/21 1:17 PM, Zied Guermazi wrote:
> hi Luis
>
> A new member function in "class gdb_disassembler" to calculate the
> instruction length only will be a good solution. In fact a big overhead
> is added by the printing of instruction disassembly, which is not needed
> at all. On aarch64, the decoder is optimized to issue many instruction
> in one trace element, and here calculating the size consumes more than
> 80% of the time. On arm, the decoder issues one instruction after
> another and here getting the size consumes 50% of the time. Considering
> the amount of traces this can sum up to a dozen of minutes in some cases
> (64MB of traces)

Indeed, that doesn't sound good.

>
> Calculating the instruction size per se, on arm is a "rapid" operation
> and consists of checking few bits in the opcode. So the time can be
> drastically decreased by having a function to calculate the size only.
>
>
> gdb_print_insn can be then changed as following (pseudo code):
>
> int
> gdb_print_insn (struct gdbarch *gdbarch, CORE_ADDR memaddr,
>         struct ui_file *stream, int *branch_delay_insns)
> {
>
>   gdb_disassembler di (gdbarch, stream);
>
>   if ( di.get_insn_size != 0)
>
>    return di.get_insn_size(memaddr);
>
>   else
>
>    return di.print_insn (memaddr, branch_delay_insns);
> }
>
> Is there a function in aarch64-tdep or arm-tdep doing job of disassembly
> ( the lower layer handling the opcode)? are we relaying on the bfd
> library for it? can someone give me a hint of where to find those functions?

The gdbarch hooks in arm-tdep.c (gdb_print_insn_arm) and aarch64-tdep.c
(aarch64_gdb_print_insn) are more like helper functions and do some
initial setup, but the code to disassemble lies in opcodes/arm-dis.c
(print_insn) and opcodes/aarch64-dis.c (print_insn_aarch64).

If you go with the route of changing "class gdb_disassembler", then
you'll probably need to touch binutils/opcodes.

If you decide to have a gdbarch hook (in arm-tdep/aarch64-tdep), then
you only need to change GDB.
>
>
> Kind Regards
>
> Zied Guermazi
>
>
> On 05.04.21 15:01, Luis Machado wrote:
>> Hi Zied,
>>
>> On 4/4/21 4:59 AM, Zied Guermazi wrote:
>>> hi
>>>
>>> I need to get the size of the instruction at a given address. I am
>>> currently using gdb_insn_length (struct gdbarch *gdbarch, CORE_ADDR
>>> addr) which calls gdb_print_insn (struct gdbarch *gdbarch, CORE_ADDR
>>> memaddr, struct ui_file *stream, int *branch_delay_insns). and this
>>> is consuming a huge time, considering that this is used in branch
>>> tracing and this gets repeated up to few millions times.
>>>
>>>
>>> Is there a lean way for getting the size of the instruction at a
>>> given address, I am using it for aarch64 and arm targets.
>>
>> At the moment I don't think there is an optimal solution for this. The
>> instruction length is calculated as part of the disassemble process,
>> and is tied to the function that prints instructions.
>>
>> One way to speed things up is to have a new member function in "class
>> gdb_disassembler" to calculate the instruction length only.
>>
>> Another way is to have a new gdbarch hook that calculates the size of
>> an instruction based on the current PC, mapping symbols etc.
>>
>>>
>>> Kind Regards
>>>
>>> Zied Guermazi
>>>
>>>
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